1. Field of the Invention
The present invention relates to the welding of aluminum containing components and more specifically, to diffusion bonding or brazing of such components.
2. Brief Description of the Prior Art
Aluminum and aluminum containing metals, i.e., alloys and the like, because of certain physical properties imparted by the aluminum are extremely attractive for fabrication into certain types of components. As is well known, aluminum's high thermal and electrical conductivity, its resistance to corrosion and its inherent reflective properties make it a particularly attractive material for use in construction and fabricating components for heating and cooling systems, heat reflecting surfaces, engine components, etc. In particular, aluminum alloys have found particular application in the fabrication of radiator panels for use in orbiting satellites.
Notwithstanding its desirable properties in many respects, aluminum and its alloys pose peculiar welding difficulties. It is well known that aluminum exposed to the atmosphere will quickly form an aluminum oxide coating which is quite refractory. Indeed, so refractory is the aluminum oxide coating that although the temperature of the aluminum or aluminum alloy may be raised sufficiently to melt the metal, the refractory skin remains as an impervious wall which encases the molten alumimum thus preventing any effective welding technique. Thus, any welding procedure involving aluminum or aluminum alloys calls for the removal of the thin, tough, transparent film of aluminum oxide which protects the surface of the metal.
In the case of aluminum brazing or diffusion bonding, wherein a braze alloy is employed, it is common practice to also use a flux which removes the aluminum oxide from the surfaces to be brazed and permits the formation of a uniform joint having good mechanical strength. However, the use of a flux is not without disadvantage since it is necessary, after the brazing or diffusion bonding, to remove the flux to avoid corrosion of the aluminum.
Another technique which has been employed in welding aluminum and aluminum alloys is to effect removal of the aluminum oxide coating just prior to the welding step. In this technique, there remains the ever present danger that oxide formation might occur before the components are welded together resulting in a non-uniform joint. Furthermore, in a diffusion process when no brazing alloy is employed, conventional techniques require extremely high pressures to effect bonding at the adjoined surfaces to an extent necessary to form a uniform joint of good mechanical strength.
The prior art as exemplified by U.S. Pat. Nos. 2,537,644 and 2,171,545 teaches the use of polymeric materials such as polystyrene and phenol-aldehyde resins as protective coatings for aluminum and aluminum alloys. However, the former patent fails to teach the coating of aluminum oxide free surfaces so as to render the surfaces amenable to fluxless brazing or diffusion bonding and the latter patent, while teaching the removal of aluminum oxide from the surface, employs an etching solution which makes the polymeric coating bond more securely to the surface.